How to remap port for public ip on Peplink Balance 20?

I have a Peplink Balance 20 with 1 wan connection and 4 clients

All the clients have the ftp port 21, but I want to be able to forward the port for example:

WAN1 Internal IP
public ip port:2221 → points to 192.168.1.2 port 21
public ip port:2222 → points to 192.168.1.3 port 21
public ip port:2223 → points to 192.168.1.4 port 21
public ip port:2224 → points to 192.168.1.5 port 21

Can somebody help and point me in the right direction, I tried different things but nothing worked.

Thanks

You need to create four port forwarding rules, an example: